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Communication via Synapses

Communication via Synapses Synapses are a fundamental part of neural pathways as they regulate decision-making in terms of exciting or inhibiting the post-synaptic neurons. Review: Action potentials (AP) reach terminal bud of the pre-synaptic neuron.

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Communication via Synapses

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  1. Communication via Synapses • Synapses are a fundamental part of neural pathways as they regulate decision-making in terms of exciting or inhibiting the post-synaptic neurons. • Review: • Action potentials (AP) reach terminal bud of the pre-synaptic neuron. • Neurotransmitters (NT), chemical messengers, diffuse across the synapse to bind with receptors on the post-synaptic membrane. • Neurotransmitters are: • Excitatory, which means they excite the post-synaptic neuron (contributing to depolarisation and propagation of the AP. • OR: • Inhibitory, hyperpolarising the post-synaptic neuron and preventing AP.

  2. Neurotransmitters can be excitatory or inhibitory • Neurotransmitters (NT) are proteins • diffuse across the synaptic cleft • bind with a receptor on the post-synaptic neuron. • Whether or not the post-synaptic neuron propagates the action potential depends on: • Which NT diffuses across • Which receptors they bind to • Which ions flow in/out of the post-synaptic neuron • Whether or not depolarisation reaches threshold • Excitatory NTs cause depolarisation • e.g. ACh, dopamine • NT binds, Na+ channels open, Na+ rushes in • Membrane potential depolarises, AP propagated • Inhibitory NTs cause hyperpolarisation • e.g. GABA, dopamine (on different pathways) • NT binds to receptor • K+ channels open, K+ rushes out • OR Cl- channels open, Cl- rushes in • Membrane potential become more negative • Action potential is prevented from propagating 0mv depolarisation threshold resting -70mv hyperpolarisation time

  3. http://www.youtube.com/watch?v=-pfG6yHAQ5U This is a useful example of an inhibitory neurotransmitter.

  4. Decision-making in the Central Nervous System (CNS) The axons of many pre-synaptic neurons feed into the dendrites of one post-synaptic neurons via synapses. The ‘decision’ whether or not to propagate the action potential along the axon of the post-synaptic neuron takes place in a region of the cell body called the axon hillock. This is achieved through summation of the incoming impulses. If the total impulse reaches threshold, the post-synaptic neuron depolarisesand the action potential is propagated. If the sum does not reach threshold, the AP is not propagated. There are two main methods of summation: temporal and spatial. action potential axon synapse axon hillock

  5. Decision-making in the Central Nervous System (CNS) • Temporal summation • Action potentials arrive in rapid succession • Depolarisation in the axon hillock is summative • If it reaches threshold before repolarisation, the AP is propagated. • Spatial summation • Action potentials arrive simultaneously from multiple sources • Some neurotransmitters are excitatory (increasing depolarisation) • Other NTs are inhibitory (hyperpolarising). • Summation in the axon hillock is summative. • If it reaches threshold the AP is propagated.

  6. How do psychoactive drugs affect the brain? • Before thinking about how drugs affect the synapses, be sure you understand how they work and are reset. • Some NTs have a normal excitatory function • Other NTs have a normal inhibitory function • In general, psychoactive drugs can: • Increase or decrease the release of NTs (e.g. THC – cannabis) • Breakdown re-uptake proteins which are responsible for returned used components of NTs to the pre-synaptic neuron (ready to use again) • Block re-uptake proteins (e.g.cocaine) • Mimic or block NTs, binding to the receptors on post-synaptic membranes • Inhibit production of new NTs

  7. Excitatory drugs increase post-synaptic transmission • Examples: • Nicotine, amphetamines, cocaine • What is the effect of cocaine? • Normal: • Dopamine acts as excitatory NT • Dopamine is re-uptaken by pumps on the pre-synaptic membrane. • With Cocaine: • Cocaine blocks re-uptake pumps • Dopamine remains in synaptic cleft • More dopamine continues to be released • Summative increase in post-synaptic transmission • Effects on mood: • Dopamine is involved in reward pathways, enhancing feelings of pleasure • Longer-lasting feelings as dopamine is not re-uptaken • Effects on behaviour: • feelings of euphoria • increased energy and alertness • highly addictive • association with depression as body reduces production of own dopamine over time

  8. Inhibitory drugs decrease post-synaptic transmission • Examples: • Alcohol, benzodiazapines, THC • What is the effect of tetrahydrocannibol (THC)? • Normal: • Dopamine release is moderated (inhibited) by GABA • With THC: • THC mimics cannabinoids and inhibits GABA release by binding to cannabinoid receptors • GABA cannot inhibit dopamine release • More dopamine is released • Effects on mood: • Dopamine is involved in reward pathways, enhancing feelings of pleasure • Not as extreme release of dopamine as with cocaine, but still higher than normal • Effects on behaviour: • intoxication • hunger • memory impairment • potential dependency

  9. http://www.youtube.com/watch?v=bCChf2WHNE4

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